Although it is clear that transferrin receptors are regulated in part by intracellular iron, it is also clear that other regulating mechanisms are involved. Since probes to the transferrin receptor gene have recently become available, we are utilizing molecular biology techniques to study the regulation of transferrin receptor expression at the molecular level. One part of this project involves the study of mouse plasmacytoma mRNA. We have found that there are two transferrin receptor mRNAs in these cells. One message lacks the untranslated part of the full-length mRNA, while retaining the coding sequences for the protein. Using this model system, we plan to study the ability of iron to regulate TfR mRNA levels at the level of the message. We will determine if the untranslated part of the message is necessary for iron regulation. Another part of the project involves the regulation of TfR gene transcription in HL-60 cells by cAMP. We have shown that cAMP shuts off transcription of both c-myc and TfR within 30' - 2 hrs after addition. We will investigate the mechanism of this shut- off and determine if the c-myc gene has any effect on transcription of the TfR gene. We have produced a TfR construct which contains only the coding region of the cDNA under LTR control. We have successfully transfected 3T3 cells with this construct and have demonstrated the existence of human TfR on their surface by immunoprecipitation and FACS analysis. We will use these cells to study the regulation of the virally controlled TfR sequence. Using these TfR viruses, we have demonstrated that HL-60 differentiation as well as the differentiation of MEL cells is altered.